The present invention relates to polyolefin films and biaxially oriented films with improved properties useable as shrink films. In particular, the present invention relates to shrink film for use in packaging lines, although the invention may also be used in other shrink film applications.
Polyolefins, polyvinyl chlorides, ionomers, polyesters, polystyrenes, and polyvinylidene chlorides have all been used in the production of shrink film. The shrinkable polyolefins currently on the market include both cross-linked and uncross-linked oriented polyethylene, oriented polypropylene, and oriented ethylene-propylene copolymers.
A shrink film's distinguishing characteristic is its ability upon exposure to some level of heat to shrink or, if restrained, to create shrink tension within the film. This ability is activated by a packager when a shrink film wrapped product is passed through a hot air or hot water shrink tunnel. Upon exposure to heat, the film shrinks around the product producing a tight, transparent wrapping that conforms to the contour of the product and which is aesthetically pleasing while providing the useful functions required of packaging materials such as protection of the product from loss of components, pilferage, damage due to handling and shipment, dirt and contamination.
Typical items wrapped in polyolefin shrink films are toys, games, sporting goods, stationery, greeting cards, hardware and household products, office supplies and forms, foods, phonograph records, and industrial parts.
The manufacture of shrink films requires sophisticated equipment including extrusion lines with “biaxial orientation” capability, mechanical center-folders, and slitters. Biaxial orientation can be accomplished with “tenter framing” stretching or “double bubble” blown film extrusion processes. The biaxial orientation causes the material to be stretched in the cross or transverse direction and in the longitudinal or machine direction. The films are usually heated to their orientation temperature range that varies with the different polymers but is usually above room temperature and below the polymer's melting temperature. After being stretched, the film is rapidly cooled to quench it thus freezing the molecules of the film in their oriented state. Upon heating, the orientation stresses are released and the film will begin to shrink back to its original unoriented dimension.
The polyolefin family of shrink films provide a wide range of physical and performance characteristics such as shrink force (the amount of force that a film exerts per unit area of its cross-section during shrinkage), the degree of free shrink (the reduction in surface area a material undergoes when unrestrained), tensile strength (the highest force that can be applied to a unit area of film before it begins to break), sealability, shrink temperature curve (the relationship of shrink to temperature), tear initiation and resistance (the force at which a film will begin to tear and continue to tear), optics (gloss, haze and transparency of material), and dimensional stability (the ability of the film to retain its original dimensions under all types of storage conditions).
In addition to the above, high speed automatic wrapping lines require that the shrink film have high film-film slip (low coefficient of friction), consistently strong static seals and higher stiffness compared to other shrink films. The high slip allows packages to freely pass one another on conveyors without sticking or clinging. The strong static seals allows for fast sealing of the packages. Finally, the higher stiffness supports high film and packaging line speeds.
Also particularly advantageous for high speed automatic wrapping lines are low shrink initiation temperature. Low initiation temperature allows faster “shrunken” film cool down due to the lower temperature. With a shorter cool down period, faster processing line speeds can be attained.
One approach to shrink films for high speed wrapping lines is a multilayer film comprising polyethylenic and polystyrenic layers. The differences between polystyrene and polyethylene layers require intervening compatabilizing layers for successful manufacture. U.S. Pat. No. 6,367,095 to Cheung et al. discloses random interpolymer useful for a compatabilizing layer. Such random interpolymers include the ethylene/styrene interpolymer resins prepared using a constrained-geometry catalyst. Unfortunately, Cheung compositions are presently expensive due to the cost of the interpolymer and may be subject to phase-out by the manufacturer.
Additional desirable characteristics of shrink films comprising polyethylenic and polystyrenic layers include: high oxygen permeability, high impact strength, high stiffness, excellent optics, printability, improved low temperature shrinkage, high capacity for additives (such as antifog additive), and radio frequency (RF) sealing.
Accordingly, it is a general object of the present invention to provide a shrinkable polyolefin film comprising polyethylenic and polystyrenic layers that will have improved properties for high speed packaging lines while not relying upon Cheung compositions for compatabilizing layers.